1. Field of the Invention
The present invention generally relates to image processing methods and, more particularly, is directed to an image processing method such as a discrete cosine transform (DCT) encoding method in which an image signal is processed in the form of a plurality of divided blocks or the like.
2. Description of the Prior Art
A discrete cosine transform (DCT) encoding method is known as a method for compressing image information, for example. In accordance with the above DCT encoding method, the image is divided into blocks (N.times.N) which are formed of N picture elements (pixels) both in the horizontal and vertical directions and the image of each block is processed in a two-dimensional DCT fashion. Then, a transform coefficient of the two-dimensional DCT is quantized. However, if quantization density is made coarse during the DCT encoding, there is then the risk that a distortion will occur on a boundary between the divided blocks.
FIG. 1 shows the case such that an image is divided into blocks (8.times.8) which are formed of 8 pixels both in the horizontal and vertical directions. In FIG. 1, an open square (.quadrature.) represents a pixel of even field constructing one frame and an open circle (.largecircle.) a pixel of odd field. Accordingly, if these blocks are processed in a two-dimensional DCT fashion, then the pixels are transformed in an unrelated fashion because pixels adjoining, for example, at the boundary of the blocks exist within different blocks. As a result, an image signal processed in a reverse transform manner is fluctuated between the blocks, thus causing a distortion to occur in the image.